Electric heating is the process of converting electrical energy into thermal energy. Since the discovery that electric current can produce thermal effects when passing through conductors, many inventors around the world have been engaged in the research and manufacturing of various electric heating appliances. The development and widespread application of electric heating, like other industries, follows a pattern: from advanced
Electric Heater
Electric Heater
countries gradually spreading to the rest of the world; from cities gradually developing to rural areas; from collective use to household and then to individual use; products evolving from low-end to high-end. In the 19th century, electric heating appliances in their infancy were mostly crude. The earliest electric heating appliances were for domestic use. In 1893, the prototype of the electric iron first appeared and was used in the United States. Then in 1909, the use of electric stoves emerged, where electric heaters were placed in the stove, meaning heating shifted from firewood to electricity, i.e., from electrical energy to thermal energy. However, the rapid development of the electric heating appliance industry truly began after the invention of nickel-chromium alloy as a heating element. In 1910, the United States first successfully developed an electric iron using nickel-chromium alloy heating wire, which fundamentally improved the structure of the electric iron and led to its rapid popularization. By 1925, products with electric heating elements installed in pots in Japan became the prototype of modern electric rice cookers. During this period, industrial electric heating products such as laboratory electric furnaces, glue melters, and heaters also appeared. The years 1910 to 1925 were a major development period in the history of electric heating appliances. In both households and industries, various types of electric heating appliances emerged and were rapidly popularized and applied, especially in households. Therefore, the invention of nickel-chromium alloy laid the foundation for the development of the electric heating appliance industry.
After the 1920s, there were not as many new application developments as in the previous period, but during this stage, all kinds of electric heating appliances were redesigned and continuously improved, marking a period of advancement in the history of electric heating appliances. In household electric heating appliances, various devices were designed to be more aesthetically pleasing, durable, and sturdy, and most were equipped with automatic temperature and time control.
Units of Measurement
1. Power: W, Kw 1Kw=3.412BTU/hr British Thermal Units/hour=1.36 (horsepower)=864Kcal/hr
2. Weight: kg: 1Kg=2.204621b (pound)
3. Flow velocity: m/min
4. Flow rate: m3/min, kg/h
5. Specific heat: Kcal/(kg℃): 1Kcal/(Kg℃)=1BTU/hr.°F=4186.8J/(Kg℃)
6. Power density: W/cm2: 1W/cm2=6.4516 W/in2
7. Pressure: Mpa
8. Thermal conductivity: W/(m℃): 1 W/(m℃)=0.01J/(cm s℃)=0.578Btu/(ft.h.F)
9. Temperature: ℃: 1F=9/5℃+32 1R=9/5℃+491.67 1K=1℃+273.15
Power Calculation
The calculation of heating power involves the following three aspects:
Power during operation
Power during startup
Heat loss in the system
All calculations should consider the worst-case scenario:
Lowest ambient temperature
Shortest operating cycle
Highest operating temperature
Maximum weight of the heating medium (maximum flow rate for flowing media)
Steps to calculate heater power:
Based on the process, draw a process flow diagram for heating (without involving material forms and specifications).
Calculate the heat required for the process.
Calculate the heat required for system startup and the time needed.
Redraw the heating process flow diagram, consider an appropriate safety factor, and determine the total power of the heater.
Determine the sheath material and power density of the heating element.
Determine the form, size, and quantity of the heater.
Determine the power supply and control system of the heater.
Working Principle
